1. Field of the Invention
The present invention relates to an active matrix type display device and a method of manufacturing the same.
2. Related Art
In an active matrix type display device, which is widely used at present, a highly heat-resistant substrate, e.g., a non-alkali glass substrate, is used as a substrate on which devices are formed during an active element forming process, and the highly heat-resistant substrate is continuously used as a supporting substrate of the display device.
When a thin film transistor is formed by using a polycrystalline silicon layer as a semiconductor layer in which a channel region is formed, the following procedure is employed, taking into consideration the processing temperature etc. at the time of forming each functional film.
First, a barrier layer is formed on a device forming substrate of non-alkali glass so that the minor constituent of the glass does not seep therefrom. Then, an amorphous silicon layer is formed thereon. Subsequently, a short-time local heat treatment using an excimer laser is performed on the workpiece so that the amorphous silicon layer becomes a polycrystalline silicon layer through the solid phase or liquid phase growth. Thereafter, the thus obtained polycrystalline silicon layer is shaped. Subsequently, a thin film to serve as a gate insulating film is deposited, and then a metal layer, on which a gate electrode and a gate wiring are formed, is formed and shaped. Furthermore, an ion implantation is performed using the gate electrode as a mask through the ion doping method in order to form a source and drain regions in the polycrystalline silicon layer. Then, a thermal processing is performed to activate ions. Accordingly, a channel region and a source and drain regions are formed in the polycrystalline silicon layer. Subsequently, an interlayer dielectric film is formed to isolate the signal lines etc. and the gate lines, and then contact holes to the source and drain regions are formed. Then, a metal layer is formed and shaped to make a source and drain electrodes, to form a thin film transistor and wirings. When an active matrix type liquid crystal display device is formed, the device forming substrate, on which the active elements are formed, is continuously used as a supporting substrate.
When a display device having a curved display surface should be manufactured, a light and flexible substrate, such as a plastic substrate, is used as a supporting substrate. Such a light and flexible substrate is preferable as a supporting substrate of a display device used in a mobile information terminal. However, since such a plastic substrate does not have a sufficient heat-resistance or a sufficient dimensional stability, it is not possible to use such a plastic substrate as a device forming substrate on which active elements of polycrystalline silicon, which are superior in switching properties, are formed, and to continuously use the plastic substrate as a supporting substrate. Accordingly, a method is employed, in which active elements are formed on a device forming substrate of glass, etc., which is highly resistant to a high temperature process, and then the active elements are transferred to a light and flexible supporting substrate formed of, e.g., a plastic. More specifically, active elements are formed on a device forming substrate of glass, the side on which the active elements are formed is bonded to a temporary substrate. Then, the device forming substrate is removed by the etching, etc., and the active elements are transferred to a final supporting substrate.
In the above-described method, however, the process of forming active elements and their wirings on a device forming substrate, which is superior in heat resistance, and then transferring them to, e.g., a plastic substrate, has a problem in the way of removing the device forming substrate used to form the active elements.
For example, in the case where the device forming substrate used to form the active elements should be completely removed, it is difficult to etch the device forming substrate without damaging the active elements on the device forming substrate.
With respect to the case where it is not necessary to completely remove the device forming substrate used to form the active elements, the present inventors have developed a display device in which the thickness of a supporting substrate of glass, on which active elements are formed, is reduced, and then the glass substrate is bonded to a flexible sheet via an adhesion layer (for example, see Japanese Patent Application No. 2002-84924). In this display device, if the flexible sheet is bent to form a curved display, it may be possible that the glass substrate breaks. Therefore, this display device is not reliable when used in a curved application.
As described above, it has been difficult to obtain a reliable and flexible display device without damaging the active matrix devices by first forming active matrix devices and their wirings on a highly heat-resistant device forming substrate, and then transferring them to a flexible plastic substrate.